1. Field of the Invention
The present invention relates to a braking stroke simulator for use in a hydraulic brake system of a vehicle, and more particularly to a braking stroke simulator operated in response to operation of a manually operated braking member.
2. Description of the Related Arts
Heretofore, there is known various hydraulic brake systems, including such a system as constituted by a hydraulic pressure control device having a pressure source for regulating hydraulic pressure discharged from the pressure source in response to operation of the manually operated braking member, to supply it into wheel brake cylinders. And, a braking stroke simulator is disposed to provide a stroke of the manually operated braking member in response to the braking operation force applied thereto.
In Japanese Patent Laid-open publication No.10-167042, which corresponds to U.S. Pat. No. 6,058,705, for example, a lengthened idle travel of pistons of a master cylinder is proposed for a hydraulic brake system, wherein a travel-dependent force is realized with simulator spring elements that act upon the pistons. For example, a compression spring is provided as a simulator spring element, and a dome-like elastomer body (rubber) is provided between a brake pedal and the pistons as another simulator spring element. It is described in the U.S. Pat. No. 6,058,705 that the elastomer body is inserted into a cylinder with initial tension (presumably meant by initial load applied to it), and the initial tension is selected to be so great that the elastomer body does not deform until a floating piston, acted upon by a stronger helical compression spring, has traversed nearly its entire idle travel.
With respect to a relationship between the braking operation force and the stroke of the manually operated braking member, it is desirable that the braking operation force required for increasing the amount of braking operation, i.e., depressing a brake pedal, is set to be greater than the braking operation force required for decreasing the amount of braking operation, i.e., restoring or releasing the brake pedal, to provide a so-called hysteresis property, which ensures a proper brake pedal feeling.
According to the hydraulic brake system as described above, when the travel of the manually operated braking member begins to produce its stroke by means of the simulator spring element made of elastomer body (rubber), the hysteris in the relationship between the braking operation force and the stroke of the manually operated braking member may be caused by the hysteresis of the elastically deformed rubber. However, when the travel of the master piston begins to produce its stroke by means of the simulator spring element made of the spring, almost no hysteresis is provided, so that the proper brake pedal feeling can not be ensured. In addition, as the initial load applied to the simulator spring element made of rubber is set independently, severe dimensional accuracy of the simulator spring element made of rubber is required for providing the initial load to be stable, which results in increase in cost. Furthermore, as the simulator spring element made of rubber is affected by aged deterioration, which will largely change the initial load applied to the simulator spring element, the brake pedal feeling as a whole might be deteriorated.